Lignocellulose-based products prepared from lignocellulosic starting materials, including products manufactured starting from vegetable fibre (e.g. wood fibre) prepared by mechanical (e.g. thermomechanical) pulping procedures, mechanical/-chemical pulping procedures (the latter often being denoted "semi-chemical" procedures) or chemical pulping procedures (such as kraft, sulfite or soda pulping), are indispensable everyday materials. Some of the most familiar types of such products include paper for writing or printing, cardboard and corrugated cardboard, as well as tissue and non-woven products.
Virtually all grades of paper, cardboard and the like are produced from aqueous pulp slurry. Typically, the pulp is suspended in water, mixed with various additives and then passed to equipment in which the paper, cardboard etc. is formed, pressed and dried. Irrespective of whether mechanically produced pulp (hereafter denoted "mechanical pulp"), semi-chemically produced pulp (hereafter denoted "semi-chemical pulp"), unbleached chemical pulp or pulp made from recycled fibres (i.e. pulp prepared from recycled paper, rags and the like) is employed, it is often necessary to add various strengthening agents to the pulp in order to obtain an end product having adequate strength properties. In the case of paper and board for use in packaging and the like, the tensile strength and tear strength under dry and wet conditions are of primary importance; moreover, notably in the case of certain grades of cardboard (e.g. so-called unbleached board for the manufacture of corrugated cardboard boxes for packaging, transport and the like), the compression strength of the material is often also an important factor.
In the field of lignocellulose-based products, considerable effort has been devoted in recent years to the development and application of strengthening/binding agents or systems which are more acceptable from an environmental and toxicity point of view than those "traditionally" used. Relevant patent literature in this respect includes the following:
EP 0 433 258 A1 discloses a procedure for the production of mechanical pulp from a fibrous product using a chemical and/or enzymatic treatment in which a "binding agent" is linked with the lignin in the fibrous product via the formation of radicals on the lignin part of the fibrous product. This document mentions "hydrocarbonates", such as cationic starch, and/or proteins as examples of suitable binding agents. As examples of suitable enzymes are mentioned laccase, lignin peroxidase and manganese peroxidase, and as examples of suitable chemical agents are mentioned hydrogen peroxide with ferro ions, chlorine dioxide, ozone, and mixtures thereof.
EP 0 565 109 A1 discloses a method for achieving binding of mechanically produced wood fragments via activation of the lignin in the middle lamella of the wood cells by incubation with phenol-oxidizing enzymes. The use of a separate binder is thus avoided by this method.
U.S. Pat. No. 4,432,921 describes a process for producing a binder for wood products from a phenolic compound having phenolic groups, and the process in question involves treating the phenolic compound with enzymes to activate and oxidatively polymerize the phenolic compound, thereby converting it into the binder. The only phenolic compounds which are specifically mentioned in this document, or employed in the working examples given therein, are lignin sulfonates, and a main purpose of the invention described in U.S. Pat. No. 4,432,921 is the economic exploitation of so-called "sulfite spent liquor", which is a liquid waste product produced in large quantities through the operation of the sulfite process for the production of chemical pulp, and which contains lignin sulfonates.
With respect to the use of lignin sulfonates--in particular in the form of sulfite spent liquor--as phenolic polymers in systems or processes for strengthening/binding wood products, the following comments are appropriate:
(i) lignin sulfonates available on a commercial scale are generally very impure and of very variable quality [see J. L. Philippou, Journal of Wood Chemistry and Technology 1(2) (1981) 199-227]; PA1 (ii) the very dark colour of spent sulfite liquor renders it unsuited as a source of lignin sulfonates for the production of, e.g., paper products (such as packaging paper, linerboard or unbleached board for cardboard boxes and the like) having acceptable colour properties.
In recent years, increasing use has been made in the paper industry of modified, polysaccharide-based substances, such as cationic starches (i.e. starches which have been modified by the introduction of cationic functionalities, normally quaternary ammonium groups). Cationic starches of the quaternary ammonium type are widely used in the industry as so-called "wet-end additives" for improving, inter alia, strength and drainage, and as binders in coatings. Other types of cationic agents which are commercially available for use as strengthening agents include cationic derivatives of guar gum [a poly(galactomannan) gum].
Reference may be made to a review by D. C. Smith in TAPPI Proceedings (1992 Papermakers Conference) pp. 393-404 for further information concerning these as well as other cationic and anionic polymeric strengthening agents ("strength additives").
By virtue of their ionic charge, such substances are able to bind relatively strongly, presumably via substantially electrostatic interaction, with oppositely charged functionalities [such as deprotonated carboxyl groups of uronic acid (e.g. glucuronic acid) moieties, or sulfonate groups originating from chemical modification of lignin] present in/on the fibres in lignocellulosic fibre pulp. However, the increase in strength achievable in this manner is determined by, inter alia, the "density" of appropriately charged groups on the surface of the fibres.
The present inventors have now surprisingly found that it is possible, by means of a straightforward procedure employing an enzyme which catalyzes the oxidation of phenolic groups (such as an oxidase classified under EC 1.10.3), in the presence of an appropriate oxidizing agent, to conjugate or graft (attach) to a lignocellulosic material (such as wood fibres or other vegetable fibres) phenolic substances (i.e. substances comprising at least a substituent containing a phenolic hydroxy group) having functionalities or substituents which in the conjugated (attached) form of the phenolic substance are, or under suitable conditions become, negatively or positively charged, respectively.
The phenolic substances in question are preferably substances of relatively low molecular weight; thus, in general, non-polymeric phenolic substances are preferred (vide infra). Phenolic polysaccharides (i.e. polysaccharides which are substituted with substituents containing a phenolic hydroxy group) are not within the scope of phenolic substances in the context of processes according to the present invention. Thus, for example, the phenolic polysaccharides employed in the context of the invention which is disclosed in applicant's International application No. PCT/DK95/00318 are not within the scope of phenolic substances employed in accordance with the present invention.
Owing to the resulting increased surface charge density, increased binding (as mentioned above) of an appropriate ionically charged strengthening agent to the lignocellulosic material may be achieved. Using the resulting product as starting material, the preparation of products, e.g. paper products, of greater strength than corresponding products prepared from lignocellulosic material with a lower content of strengthening agent may be achieved.